A report in The Observer this week said that Sorrell, the WPP chief executive, could launch his bid in days, bringing an end to TNS boss David Lowden's plans for a pan-European market research merger.
However, there is a note of caution at WPP as Sorrell considers the implications of a possible advertising recession in Europe and the US, where growth is slowing. Sorrell is also concerned about paying over the odds for TNS, which has agreed a nil-premium merger with Gfk.
However, TNS is growing fast in emerging markets, particularly in parts of Asia where WPP is also pouring growth efforts into.
The paper said that despite this caution Goldman Sachs, WPP's financial adviser, was working on the final details of a bid document.
However, in an interview in the Financial Times today, Sorrell struck a cautious note and aired concerns about the acquisition and non-organic growth.
He said: "If we walk away, where would the TNS share price be? My guess is it would go back [from 228p at Friday's close] to 160p-170p.
"It's no accident that McKinsey and Goldman Sachs are very strong, because they've grown organically. The model we, Omnicom, Publicis and Interpublic have is the most difficult to manage, because you have different tribes [and] warring factions that work in isolation."
As Sorrell considers his next move, a crucial vote by Gfk's largest German shareholder is to be delayed. A group of German councils, the Verein, which owns 58% of the firm, was due to vote on Friday but has now delayed that vote by three weeks until July 21 amid signs of opposition.
The Sunday Times reported that one of the councils concerned, Nuremberg, has already decided to vote against the merger over fears of job losses.
However, any vote the councils take can be overridden by Verein's board of directors, which fully backs the merger.
Shareholders in TNS are due to vote on the merger on July 18.